1. Field of the Invention
The present invention relates high density polyethylene (HDPE) useful for insulating wire and cable. More particularly, the HDPE resins of the invention are useful for telecommunication cables and, by virtue of their unique combination of melt elasticity and short-chain branching characteristics, have improved strippability and improved oxidative stability upon exposure to water-blocking cable filler compounds.
2. Description of the Prior Art
HDPE is widely used as the insulation material for various types of wire and cable. The resin can be extruded onto the metal conductor as a single layer, two layers as with “foam/skin” constructions or three layers as with “skin/foam/skin” constructions. In foamed applications the foamed polymer is surrounded with a thin outer layer of solid polymer or “skin.” Foamed constructions are particularly advantageous for telecommunication applications since the inner foam layer decreases the electrical capacity of the overall insulation which allows closer spacing of the insulated conductors in the telephone cable. The foam and skin layers are applied using high speed coextrusion processes.
Telephone “singles” are produced by extrusion coating 19, 22, 24 or 26 AWG copper wire with either solid or foam/skin HDPE insulation to a thickness of from 5 to 15 mils. These are commonly uniformly twisted into pairs and 25 or more pairs are bundled in a metallic or plastic sheath to produce the telecommunication cable. The exterior jacket provides mechanical protection for the individual conductors within.
In addition to other requirements, the insulated wires used for telecommunications must meet certain thermal oxidative stability and insulation adhesion standards. Insulation adhesion is important since it determines the “strippability” of the wire, i.e., the amount of force necessary to strip the insulation from the conductor. Standards published by the Insulated Cable Engineers Association, Inc., for example, specify that adhesion of insulation to a 24 AWG conductor should be such that a force not exceeding 3 lb/ft (13 Newtons) is required to strip the insulation from the conductor when tested in accordance with ASTM D 4565. Strip force standards are also published for other gauge wires.
Thermal stability requirements are based on the correlation of accelerated testing studies with field experience and, in this area, the oxidative induction time (OIT) test is generally recognized as the industry standard.
Stabilizers are incorporated into the HDPE insulation to provide oxidative stability. Combinations of primary antioxidants of the phenolic type and metal deactivators are typically employed to protect against oxidative degradation of the HDPE. Combinations of pentaerythrityl tetrakis[3(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate] (IRGANOX 1010) and N,N′-bis[3,(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionyl]hydrazine (IRGANOX MD 1024) have been shown to be effective. A discussion of primary antioxidant/metal deactivator combinations is provided in Chapter 2 of the Plastics Additives Handbook, edited by R. Gächter and H. Müller, Hanser Publishers (1987), and in an article by G. D. Brown, International Wire and Cable Symnposium Proceedings 1987, pp. 337-343.
A second type of degradation, referred to as “treeing,” is also known to occur with polyolefin insulated wires and cables. This type of deterioration is caused by moisture and, to prevent or minimize this problem, various water-blocking filling compounds are forced under pressure into the telecommunication cable cores to surround the individual insulated conductors and fill the voids and interstices therein. The water-blocking filling compounds are usually hydrocarbons of a heavy oil or waxy constitency. While these cable fillers have generally proven to be effective water-blocks, they have a tendency to extract the stabilizer(s) and thus, in time, reduce oxidative stability of the insulation materials. Whereas the oxidative stability of the insulation may be initially adequate, upon exposure to the water-blocking agent for a period of time, there can be a significant decrease in stabilizer protection which can lead to premature catastrophic failure.
To overcome this problem and provide improved oxidative stability when insulated conductors are exposed to water-blocking fillers work has focused on developing improved stabilizer packages where combinations of specific antioxidants are employed.
It would be highly useful if HDPE insulation resins were available which exhibited improved oxidative stability upon exposure to water-blocking fillers. It would be even more advantageous if the insulation compositions also had improved strippability. These and other advantages are obtained with the improved HDPE resins of the invention and insulation compositions prepared therewith which are described in detail to follow.